Alright. You probably have the porche design them installed. It overwrites the stock theme so you won't be able to delete it normally. Go to application management(just search it on your blackberry) and find the porche design theme there. Select it then delete it after. Then your device should restart and you're back to normal! Easy as that.

Hello I had the same problem, this is what you do...
1) Download and install BBSAK
2) Plug in your device
3) Click Read System
4) Find the Porch Design theme, its should be called something like net_rim_theme_PorcheDesign
5) Then Click the file
6) Click Remove button to your left
7) Your device should now reboot and youll have the default theme back.

Note: You can only have the default theme or the Porche Theme, you cannot have both.